The present invention relates to the test method of the A/D converter, especially that of the dynamic characteristics of the A/D converter.
Recently, demand for high-speed A/D converter has been increasing rapidly for processing of digital signals such as a video signal, the recording of the square wave and measurement in general. Under such circumstances, the importance of the test method used for testing the dynamic characteristics of an A/D converter in operation is increasing. Sometimes the input analog frequency close to or higher than the Nyquist frequency is used, and, in such case, there is the need for the efficient measurement of the dynamic characteristics of the A/D converter which is dependent on the input analog frequency.
According to conventional test methods, as described in "Tests Unearth a-d Converter's Real-World Performance" by Neil, M. and Muto, A., published in Electronics, Vol. 55, No. 4, pp. 127-132, Feb. 24, 1982, and in an article entitled "Increasing Need for the Research and Development of High-Speed A/D Conversion IC in Parallel with the Spread of Video Processing Technology" in Nikkei Electronics, No. 338, pp. 137-155 (1984).
FIG. 2 shows one of the typical conventional test methods discussed in the above-mentioned technical literatures. In this figure, 1 denotes the sine wave generator, 2 denotes a A/D converter under test, 3 a D/A converter, 4 a oscilloscope and 5 denotes a clock generator. The sine wave output of the sine wave generator 1 is inputted to the analog input terminal of the A/D converter 2 to be tested. The A/D converter 2 under test is synchronized with the conversion clock generated by the clock generator 5, and outputs the digital data. The digital data is converted again into the analog signal by the D/A converter 3 whose resolution and the conversion speed are higher than those of the A/D converter 2. In this case, the analog signal can be regenerated by selecting the correlation between the signal frequency f.sub.SG generated by the sine wave generator 1 and the conversion clock frequency f.sub.SPL of the clock generator 5. As seen from FIG. 3, the envelope waveform of the input sine wave can be regenerated by selecting the relationship f.sub.SG =f.sub.SPL /2+.DELTA.f, and the beat waveform can be regenerated at the frequency .DELTA.f by selecting the relationship f.sub.SG =f.sub.SPL +.DELTA.f. The dynamic characteristics of the A/D converter can be tested by observing said regenerated waveform by the oscilloscope 4. In the case of the conventional test method, however, the sine wave is used as the input test wave, so that the inclination of the waveform or the slew rate at each value of the amplitude of the waveform varies by the individual amplitudes. Thus, the slew rate is especially small near the peak of the wave, and is equivalent to that at the time of the d-c input, so that the dependency of this part on the input signal frequency could not be tested adequately. Besides, testing the band width of the analog input requires a complicated procedure such as the sequential sweep of f.sub.SG and the detection of the analog input frequency at which the amplitude of the reconstructed waveform falls by 3 dB.